With respect to separation of a mixture, there are various techniques for removing a substance (for example, a salt) dissolved in a solvent (for example, water) and the use of membrane-separation methods has been expanding as a process for saving energy and resources in recent years. Examples of a membrane used in the membrane-separation methods include a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, and a reverse osmosis membrane and these membranes are used for obtaining drinking water from, for example, seawater, brine water, or water containing a harmful substance, producing industrial ultrapure water, treating effluent, and recovering valuables.
Most commercially available reverse osmosis membranes and nanofiltration membranes are composite semipermeable membranes, which are divided into two types: one having, on a support membrane, a gel layer and an active layer obtained by the cross-linking of polymers; and the other having an active layer obtained by polycondensation of monomers on a support membrane. Among them, a composite semipermeable membrane obtained by coating a support membrane with a separation functional layer comprising cross-linked polyamide obtained by polycondensation of a polyfunctional amine and a polyfunctional acid halide is widely used as a separation membrane having a high permeability and a high selective separation performance.
In a water distillation plant using a reverse osmosis membrane, a higher water permeation performance is demanded for further reduced running cost. As a solution to such a demand, a method in which a composite semipermeable membrane provided with cross-linked polyamide polymers as a separation functional layer is brought into contact with an aqueous solution containing nitrous acid is known (patent literature 1). The water permeation performance can be improved by this treatment while the boron removal rate before the treatment is maintained. However, the resulting water permeation performance is not sufficient.
One factor that affects the water permeability of a composite semipermeable membrane is the structure of protuberances. It is proposed to enlarge the protuberances and thereby increase the substantive membrane area and water permeability (patent literature 2). In this proposal, addition of various additives at the time of interfacial polymerization results in larger protuberances and increased water permeability, but there is a concern about lowered desalination rate.
The water permeability of a composite semipermeable membrane seems to be affected by not only the water permeability of the separation functional layer itself but also that of the support membrane. In order to improve the water permeability of the support membrane, a support membrane produced from a solution of mixed hydrophilic macromolecules has been developed (patent literatures 3 and 4). However, in such hydrophilic support membrane, the hydrophilic macromolecules concentrate near the separation functional layer side and form a dense layer, which causes insufficient formation of a protuberance structure and results in a problem of decreased water permeability.